Photovoltaic cells generate an electric current (photocurrent) when exposed to light. Thin-film photovoltaic (TFPV or, herein, simply “PV”) panels (also called “modules”) include PV cells and are commonly used to produce electricity from solar or other illumination. An exemplary PV panel includes a stack of a substrate, a transparent electrode layer on the substrate, a p-i-n semiconductor diode layer, and an opaque (e.g., reflective) electrode layer. Since conventional PV photodiodes develop relatively low voltages under illumination, the layers are divided, e.g., by laser scribing. Rather than the single panel being a single, large-area photodiode (very high current; very low voltage), areas of the diode layer (“cells”) are separated from each other and connected in series by the conductive layers to form multiple spatially-separated area photodiodes (medium to high current; high voltage). For example, dividing a panel into ten series-connected cells would reduce the current to approximately 10% but increase the voltage by 10×.
TFPV panels as manufactured can contain defects, e.g., of sizes from 10-100 μm. These defects can provide current paths between the two electrode layers. Although the defects themselves are small, since TFPV panels use conductive and semiconductive layers spread over areas of the panel, a small defect can be electrically connected to a much larger area of the panel than the defect itself. For example, a shunt defect that electrically connects layers 120 and 140 (FIG. 1) can sink current generated across an entire cell 105. This can modify the operating point (e.g., voltage) of other portions of the same cell away from the maximum power point, thereby lowering the output power produced by the cell. Moreover, for large enough shunts, the shunted portion of the cell can consume the power output of other portions of the cell. Therefore, even a few large shunts can have a significant impact on the overall module efficiency (and yield), and contribute significantly to the gap between cell and module efficiencies.
Various schemes have been described for module scribing. For example, “Novel series connection concept for thin film solar modules” by Haas et al. (Prog. Photovolt: Res. Appl., 2012; DOI: 10.1002/pip.2188) describes pointwise contacts between series cells instead of linewise contacts, but this does not reduce the negative effects of a shunt on its particular cells.
U.S. Pat. No. 4,640,002 to Phillips et al. describes using a laser to scan a reverse-biased PV panel to locate shunt defects. The semiconductor layer is scanned before a permanent second electrode is formed over that layer. The defect is then eliminated by applying localized high current to cause it to burn open, or by irradiating the shunt with a laser to burn the shunt open. However, this requires precise detection of the shunt locations and precise application of radiation to open the shunts. Moreover, larger shunts than the laser spot must be opened by successive laser shots spread over the shunt, which can consume a great deal of time in manufacturing. Also, this scheme operates on the semiconductor before a top electrode is formed, so shunt defects formed or induced by the top-electrode deposition or formation process cannot be detected in this way.
U.S. Pat. No. 8,231,431 to Gajaria et al. describes locating shunts optically and opening them by applying reverse bias to cells including shunts. However, application of reverse bias can damage PV cells. U.S. Pat. No. 8,318,240 to Zapalac et al. describes passivating or mechanically removing solar-cell structures in the vicinity of a defect, but this reduces the area available to generate current.
Accordingly, there is a need for ways of reducing the negative effects of shunt defects (e.g., power loss and localized heating) without significantly reducing the area available to generate power. It is also desirable that this need be met without significantly increasing the time required to manufacture a TFPV panel, and without increasing the probability of damaging a PV cell while opening the shunt.